1. Field of the Invention
The invention relates to air flow meters and more particularly to ion drag air flow meters for automobile applications.
2. Description of the Known Technology
Automotive emission control systems often use an air flow meter to provide vital information relating to the amount of air drawn into an engine of an automobile. Typically, the air flow meter includes an element located in an interior chamber of the air flow meter. This element is an electrically conductive element that is heated as current flows through this element. As air flows across this element, the element cools and the electrical resistance changes. This change in resistance thus gives a signal that is proportional to the air flow.
However, air flow meters of this type, sometimes called “hot wire” air flow meters, have several drawbacks. Air flow meters of this type are not bi-directional. More specifically, air flow in either direction cools the element, thus changing the resistance. This change in resistance only indicates that the element has cooled and does not indicate which direction the air cooling the element is flowing. Second, these types of air flow meters have a relatively slow response time, generally in the order of fifteen milliseconds. Third, this type of air flow meter does not respond consistently to non-laminar air flow since the element only samples a portion of the air in the entire meter.
Another type of air flow meter is an ion drag air flow meter. This type of air flow meter uses a high voltage power supply to create a corona discharge by ionizing air around a highly curved surface, typically the tip of a needle. If the electrical gradient between this highly curved surface and a collection electrode is sufficient, electrons are released from the highly curved surface and collected by the collection electrode. Ions released by the highly curved surface travel towards the electrode. The travel of these ions is altered by air flow in the same space and as a result, the ions collected by the electrode provide a signal indicating the flow of air through the air meter.
Ion drag air flow meters have advantages over “hot wire” air flow meters in that ion drag air flow meters, if configured properly, can measure air flow in either direction. Additionally, ion drag air flow meters have quick response times and can respond to non-laminar air flow. However, previous designs of ion drag air flow meters have generally been disfavored over “hot wire” air flow meters, even with the noted advantages, because ion drag air flow meters have issues including stability, accuracy, insufficient dynamic range, complexity and cost that are not present in “hot wire” air flow meters.